Nowadays, conversion of methane to more valuable products is of paramount importance, due to the existence of large gas resources throughout the world. Oxidative coupling of methane (OCM) to C2 hydrocarbons (e.g. ethane and ethylene) is a well-known conversion process. The non-selective gas-phase reaction, however, leads to low C2 selectivity and yields.
Several studies have been carried out in packed-bed reactors in co-feed operation mode according to which methane and oxygen were fed to the reactor at the same time. The results, however, were not so promising due to the low C2 selectivity, which was caused by the fact that the oxidant of the process is gaseous molecular oxygen.
To overcome this problem, researchers have tested using perovskite membrane reactors, which have led to the indirect mixing of methane and oxygen during their transport. The major advantage of membrane reactors is preventing the direct mixing of oxygen and methane. This is because the perovskite membrane allows the permeation of ionic oxygen species produced under the operating conditions of the reaction, and keeps methane on the other side. Once the permeated ionic oxygen species reach the methane side, they readily react with the methane that is always in excess amounts due to the transportation mechanism of oxygen. This helps avoid the, side reaction of methane combustion, increasing the selectivity and, to some extent, the yield of the OCM reaction.
Oxygen permeable perovskites that can be used for this purpose are known to have the general formula of ABO3 in which A and B are of rare and alkaline earth metal ions and transition metal ions respectively.
Substitution of alkaline-earth ions on the A-site affects the oxygen nonstoichiometry of the perovskite, while B-site is known to help optimize the catalytic properties of the perovskite-type oxides for oxidation reactions.
Dense membranes of the type of LaxSr1-xCoyFe1-yO3-δ are conductors of both oxygen ion and electron.
La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) powders that are commonly used as membrane reactors, are prepared through complexation methods using ethylenediamine tetraacetic acid (EDTA) and an organic acid buffer, which can be later combusted, leaving no traces in the catalyst structure.
According to Pingying Zeng et al (J. of Mem. Sci. 302 (2007)), stoichiometric quantities of the desired metal salts are added to an EDTA, NH4OH aqueous solution under heating and stirring, and then followed by the addition of citric acid. The pH value of the system is controlled around 6. This is because at lower pH values EDTA precipitates, leading to the formation of non-homogenous La0.6Sr0.4Co0.8Fe0.2O3-δ powders. The water content of the reaction mixture is then evaporated to yield a dark purple gel, which is then pretreated at 250° C. for several hours to form a solid precursor, which is then calcined at 800° C. for 5 h to obtain the oxide with the desired composition.
The La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) powders prepared through such conventional methods, however, are found to suffer the disadvantage of relatively low C2 hydrocarbon selectivities and yield if used in OCM reactions.